1. Field of the Invention
The present invention relates to a method of forming a metallic electrode of a semiconductor device through patterning a metallic film by a cutting work. The present invention also relates to a metallic electrode formation apparatus.
2. Description of Related Art
As a low-cost formation technique of a metallic electrode for solder connection, Patent Document 1 describes a technique for forming a metallic electrode without conducting a photolithography process in patterning.
According to Patent Document 1, a semiconductor device includes a bed electrode formed on one surface of a semiconductor substrate, a protective film formed on the bed electrode, an opening formed in the protective film, and a metallic electrode for connection formed on a surface of the bed electrode exposed in the opening. The metallic electrode of the above semiconductor device is formed through forming a metallic film on the bed electrode and the protective film, and pattering the metallic film by a cutting work, which utilizes a difference in level between the surface of the bed electrode exposed in the opening and an upper surface of the protective film. More specifically, the difference in level is such a depression that the surface of the bed electrode exposed in the opening recedes from the upper surface of the protective film.
In formation of the above type of semiconductor devices, a part of the protective film is removed together with the metallic film above the protective film in order to reliably remove the metallic film located above the upper surface of the protective film even when the semiconductor substrate has a thickness variation. However, in order to ensure an electric insulating property of a semiconductor element in a semiconductor substrate and in order to suppress a variation in electric insulating property in a surface of the semiconductor substrate, it is necessary to ensure that the protective film has a predetermined thickness. Thus, when a metallic electrode is patterned by a cutting work, it may be preferable to conduct the cutting work with high accuracy in amount of cutting to an extent that, in the whole surface of the semiconductor substrate, a variation in amount of cutting with reference to a surface of the metallic film falls within +/−1 μm (2 μm or less). For example, when a bed electrode made of aluminum is formed to have a 5 μm thickness and when the protective film is formed to have a 10 μm thickness from one surface of the semiconductor substrate, the protective film has an approximately 8 μm thickness above the bed electrode. Therefore, the accuracy of 2 μm or less (within +/−1 μm) is required to ensure an approximately 3 μm film thickness of the protective film above the bed electrode, where the approximately 3 μm film thickness may be a minimum thickness required to ensure electric insulating reliability.
To conduct the cutting work, a semiconductor substrate is adsorbed and fixed on an adsorption stage. In this case, because the rear surface of the semiconductor substrate is deformed into a flat surface, the principle surface of the semiconductor substrate is deformed into a shape reflecting an original irregularity of the rear surface. The cutting work is typically conducted along a plane parallel to the adsorption stage. Therefore, if the semiconductor substrate has, with respect to the plane, a thickness variation larger than the above-described required accuracy (within +/−1 μm) in amount of cutting (cutting amount), there may exist a region where the required accuracy (+/−1 μm) in cutting amount is not satisfied. In this case, a yield rate is decreased.
In view of the above, the applicant of the present application has proposed a method described in Patent Document 2. According to Patent Document 2, after the semiconductor substrate having the metallic film is adsorbed and fixed on the adsorption stage, a surface shape measurement device acquires data about a surface shape of a surface part of the semiconductor substrate. In the above, the surface part is a part of the metallic film that covers the protective film. Then, based on the data about the surface shape, a deformation device deforms the semiconductor substrate by applying displacement to the semiconductor substrate from an adsorption stage side of the semiconductor substrate so that a distance between the surface part of the semiconductor substrate and a cutting plane, which is set parallel to the adsorption stage, falls within a predetermined range. Then, the surface shape measurement device measures the surface shape of the deformed semiconductor substrate. When it is determined that the distance between the surface part and the cutting plane is in the predetermined range, the cutting work is conducted along the cutting plane while the semiconductor substrate deformed by the deformation device is being adsorbed and fixed on the adsorption stage. As described above, since the cutting work is conducted in a state where irregularity of the surface part of the semiconductor substrate has become smaller, it is possible to improve the yield rate.
Patent Document 1: JP-2006-186304A
Patent Document 2: JP-2009-49356A corresponding to US-2008/0217771A
However, Patent Document 2 fails to specifically address an arrangement of multiple actuators acting as the deformation device. Therefore, depending on the arrangement of the multiple actuators, it is likely that the irregularity of the surface part of the semiconductor substrate cannot be corrected with high accuracy. That is, undulation of the semiconductor substrate cannot be corrected with high accuracy. In order to correct the undulation of the surface of the semiconductor substrate with high accuracy, it may be preferable to decrease a pitch of the actuators. However, the decrease in the pitch leads to an increase in the number of actuators. In accordance with the increase in the number of actuators, a control system is complicated and production cost is increased.
According to a known sampling theorem, when the actuators are arranged at a pitch less than or equal to one-half of wavelength corresponding to spatial frequency of the undulation of the surface of the substrate, the undulation can be recovered by a chain of the displacements of the actuators. That is, it is conceivable that when the actuators are arranged ideally based on the sampling theorem, an upper limit of the pitch of the actuators capable of correcting the undulation with high accuracy is one-half of wavelength of spatial frequency of the undulation of the semiconductor substrate.